The invention relates to a method and a control device for protection time setting in an electric drive system comprising an electric machine and a pulse-controlled inverter in particular when controlling semiconductor switches of a pulse-controlled inverter.
Electric machines with a pulse-controlled inverter are used in hybrid vehicles, for example, where they are optionally operated in the motor or generator mode. In the motor mode, the electric machine generates an additional driving torque that supports the internal combustion engine in an acceleration phase, for example; in the generator mode, it generates electrical energy that is stored in an energy store, such as a battery or a super-cap, for example. The operating mode and power of the electric machine are set by means of the pulse-controlled inverter.
Known pulse-controlled inverters comprise a series of switches used for optionally switching the individual phases of the electric machine relative to a high potential, the so-called intermediate circuit voltage, or relative to a low reference potential, in particular ground. The switches are controlled by an external control unit which calculates a desired operating point for the electric machine depending on the driver's desire (acceleration or breaking). The pulse-controlled inverter is connected to the control unit and receives the corresponding operating data and/or control commands from said control unit.
In the case of a disturbance or a fault, for example in the event of an excessively high battery current or an excessively high supply lead current, the pulse-controlled inverter is switched to a safe state in order to prevent possible damage to electrical components. In this case, various switching states can be realized in conventional methods.
By way of example, all switches connected to the low potential so-called low-side switches can be closed and all switches connected to the high potential, so-called high-side switches can be opened. This operating mode is also designated as a short-circuit state with respect to low potential. Alternatively, it is also possible for all the high-side switches to be closed and all the low-side switches to be opened, thus giving rise to a short-circuit state with respect to high potential. In a different turn-off method, all switches of the pulse-controlled inverter are opened. This is also designated as a freewheeling mode.
The document DE 10 2006 003 254 A1 discloses for example a combination of turn-off methods: since for example the phase current can still rise for a short time after the switch-over to the short-circuit state, said document proposes using both known turn-off operating modes sequentially and switching the electric machine firstly into the freewheeling mode and then into the short-circuit state.
The freewheeling mode is activated in each case for a specific protection time, such that on account of switch-off delays or residual voltages it is always possible to ensure that no short circuit between high and low potentials occurs. This protection time is subject to certain tolerances in the case of a setting via hardware, for example on account of temperature fluctuations or fluctuations governed by the operation duration. In contrast thereto, in the case of setting of the protection time via the control software of pulse-controlled inverters a precise setting can be effected, although the software cannot react to fault reactions in the hardware.
The document DE 94 13 274 U1 discloses a gate array for the pulse width modulation with control signals for pulse-controlled inverters with a microprocessor for generating PWM signals and devices for dead time compensation, for DC component suppression and for latching.
There is a need for solutions which can ensure the safety and robustness of an electric drive system in all fault cases, in particular in the event of a fault-induced transition between different operating states of a pulse-controlled inverter.